The present invention provides an exhaust gas treatment unit for the selective catalytic reduction of nitrogen oxides under lean exhaust gas conditions which contains at least one catalyst with catalytically active components for selective catalytic reduction (SCR components).
Stationary combustion units in power stations and diesel engines and also lean operated gasoline engines emit an oxygen-rich exhaust gas which contains, as harmful substances, unburnt hydrocarbons (HC), carbon monoxide (CO) and nitrogen oxides (NOx). In order to remove nitrogen oxides from lean exhaust gases, use of the so-called selective catalytic reduction (SCR) process has been applied for a long time in the power station sector. Here, ammonia (NH3) is supplied to the exhaust gas and this reacts selectively, on a suitable catalyst, with nitrogen oxides to give nitrogen and water. Under stationary operation, conversion rates of more than 80% can be achieved for nitrogen oxides in this way.
As a result of the legally required reduction in nitrogen oxide emissions from motor vehicles (EURO IV and EURO V in Europe by 2005 and 2008 respectively, LEV II in USA by 2007), the application of SCR technology, in particular in the case of heavy duty vehicles, is becoming more and more urgent. Due to the high toxicity and volatility of ammonia, non-toxic precursor compounds, which release ammonia under appropriate treatment, are used for mobile applications in motor vehicles. Urea in the form of an aqueous solution is preferably used. To hydrolyze urea, so-called hydrolysis catalysts which hydrolyze urea solution to give ammonia and carbon dioxide in a two-step reaction are used. The hydrolysis catalysts are located upstream of the SCR catalyst in the exhaust gas treatment unit. The urea solution is injected into the exhaust gas stream upstream of the hydrolysis catalyst with the aid of a special metering system.
The disadvantage of this type of exhaust gas treatment is the fact that both hydrolysis of urea and also selective catalytic reduction with modem SCR catalysts is initiated only above 160 to 200° C. Therefore, under operating conditions of the internal combustion engine with exhaust gas temperatures below this temperature range, nitrogen oxides produced by the engine pass through the exhaust gas treatment unit in unchanged form and are released into the environment. Modern urea metering systems take this fact into account and inject the urea solution into the exhaust gas stream only when the exhaust gas temperature is above 170° C.
An object of the present invention is to provide an exhaust gas treatment unit which is able to reduce the emission of nitrogen oxides from lean operated internal combustion engines even under operating conditions with exhaust gas temperatures below the light-off temperature for the hydrolysis of urea and for selective catalytic reduction.
A further object of the invention is to provide a process for removing nitrogen oxides from the lean exhaust gas from an internal combustion engine by selective catalytic reduction using ammonia.